1. Field of the Art
The present invention relates to a device for controlling a direct current motor so that the motor is operated at a desired constant speed.
2. Related Art Statement
In the art of DC motors, there is a general recognition that variations in load exerted on a mechanical component connected to a DC motor, and in linearity of a control circuit of the DC motor, make it difficult to employ an open-loop control system for operating the DC motor at a constant speed based on a commanded value selected from among plural command values of speed.
To cope with the above difficulty, there has been employed a feedback control system which employs an encoder to detect a current speed of the DC motor. The encoder generates a feedback signal which has a period corresponding to the detected speed of the DC motor. The feedback signal is convered by an F/V converter into a feedback voltage which is compared with a reference voltage which is determined by a commanded speed value. The DC motor is controlled so that a difference between the feedback voltage and the reference voltage becomes zero. Thus, the DC motor is operated at a selected constant speed.
Problem Solved by the Invention
However, the above-indicated feedback control system permits the DC motor to operate only at predetermined plural command speeds. In other words, it is not possible to operate the DC motor at a speed which is intermediate between the adjacent command speeds, i.e., at a speed other than the predetermined command speeds. Although increasing the number of command speeds will make it possible to operate the DC motor at a speed near a desired speed, this solution leads to increased number of speed command data and requires a complicated control system.